Olefins, especially .alpha.-olefins, are often polymerized by the aid of a catalyst composition, in which the catalyst is formed from a compound of a transition metal of the Groups IV-VI of the periodic table and a compound of a metal of the Groups I-III of the periodic table and activating it by reduction, i.e. the co-catalyst. The so-called Ziegler-Natta-catalyst has been further developed by using as the support for the transition metal an inert carrier, to which the transition metal compound is layered to improve the activity of the catalyst when it is catalyzing the polymerization reaction.
Yet, influenced by this catalyst composition, the asymmetrical olefin monomers often polymerize to various kinds of stereoisomeric polymers to form mixtures of e.g. isotactic, atactic and syndiotactic polymers, from which the desirable stereoisomer must be separated by means of often troublesome washing and other stages. When mainly a polymer of a certain stereospecific form is desired, e.g. an isotactic polyolefin from an asymmetrical olefin monomer, the influence of the catalyst on the stereospecifity may be improved by adding a donor compound to the catalyst. Because of a certain steric structure, the donor compound contributes to the settling of the monomer molecule in a certain position in the catalyst particle at the end of the growing polymer molecule, thus giving the molecule chain of the polymer a certain stereoisomeric structure and making the polymer product obtained, dependent on the donor compound selected, more or less as desired. There are two possibilities to add a donor to the catalyst: 1) a so-called internal donor is added to the catalyst mixture of the transition metal compound and the carrier or 2) the donor is added to the mixture of the monomer and the catalyst component in the polymerization reactor only when the cocatalyst is added, whereby the donor is called an external donor. A donor compound can, of course, also be used in both stages, whereby the donor can be a similar or a different compound at the various stages.
To asymmetrical i.e. stereospecifically polymerizing monomers except ethylene in which all side groups of the two non-saturated carbon atoms are hydrogens, and the rare case, where all side groups are similar, e.g. tetramethyl ethylene. A certain stereospecific form is made desirable by the fact that the properties of the polymer obtained for a certain purpose are more favorable, e.g. the isotactic polyolefins crystallize more easily, their bulk density is greater, their mechanical properties are better; thus they are e.g. stronger, etc. The adhesiveness of the atactic form, i.e. the adhesion properties, are generally better than in the tactic forms and they are thus suitable e.g. for glue applications.
When polymerizing asymmetric olefin monomers, i.e. the groups attached to carbon atoms by non-saturated bonds being of different kind at least as far as one group is concerned, the catalyst can comprise a compound improving the stereospecifity of the catalyst, i.e. an electron donor, which, as a donor of an electron, easily can attach to the resting structure of the catalyst and due to its steric influence direct the monomer molecule, which is going to attach to the polymer chain, to a position where the polymer molecule produced is, in a way, stereospecific as to its structure. Among such donors are numerous organic compounds, including esters, carboxylic acids, alcohols, ketones, aldehydes, nitriles, amides, amines, organic phosphorus and silicon compounds, etc. These compounds also have other influences on the properties of the catalyst, e.g. the activity of the catalyst varies depending on the donor used. If the donor is an ester of carboxylic acid, the esters of aromatic acids, benzoates, phthalates, toluates, anisates, etc. are used with the most preferable being dialkyl phthalates.
Combinations of various donors are also known in the art. Thus, from the JP publications 59172507, 59206409, 59206415, 59206416, 59206424, 60262804, 61103910 and 61108614 is known a catalyst composition, which is prepared from MgCl.sub.2, 2-ethylene hexyl alcohol, titanium tetrachloride, di-isobutyl phthalate and in some cases from phthalic acid anhydride.
The patent family comprising the publications DE 3,540,699, EP 226,003 and U.S. Pat. No. 4,761,461, discloses a catalyst composition, which has been obtained by bringing together MgCl.sub.2, alcohol, such as ethanol, an ester of phthalic acid, such as di-isobutyl phthalate, and titanium tetrachloride in certain conditions. The ester of phthalic acid can be added either at the stage when MgCl.sub.2 and ROH are reacting with each other, or at the latter stage when the reaction product of MgCl.sub.2 and alcohol are reacted with titanium tetrachloride.
The high titanium content of the above-mentioned catalysts causes color problems in the polymers and the high phthalic acid content causes non-desirable aromatic residues in the polymers.